1. Field of the Invention
The present invention relates to a method of correcting a predetermined adjustment value for an image forming apparatus that forms an image of each of separated colors based on the adjustment value, an image forming apparatus using the method, and a recording medium for realizing the function of the image forming apparatus. In particular, the invention relates to a method of efficiently correcting misregistered color images formed on a carrier.
2. Description of the Prior Art
An image forming apparatus such as a digital color copying machine or the like performs image processing on input data for each color component, and then forms a multi-color image by layering the images of the color components. If the images of the color components are misaligned in forming the multi-color image, color misregistration occurs in the resulting multi-color image, which may deteriorate the image quality. Particularly, in the image forming apparatus provided with an image forming portion for every color component in order to improve the speed of multi-color image formation, the images of the color components are formed at respective image forming portions and then layered one after another so as to form the multi-color image.
In such an image forming apparatus, misalignment often occurs in transfer positions of the images of the color components, causing a significant problem of color misregistration in the resulting multi-color image. In order to solve such a problem, a conventional image forming apparatus performs color adjustment corrects color misregistration in a multi-color image for achieving precise alignment of different color component images so as to form a good multi-color image without color misregistration. The color adjustment is usually performed by detecting a displacement of an image forming position of a color component from an image forming position of a base color component by an optical sensor. An amount of correction is then determined based on the detected result, and the timing for forming the image of each color component is adjusted in accordance with the amount of correction such that the transfer positions of the color component images match with one another.
To determine the amount of correction, first, a method of transferring the images of different color components at the same timing and then detecting the distance between the transfer positions of the color components is disclosed. Moreover, the second method is disclosed by which the images of different color components are transferred at the same timing and the density of the multi-color image formed by layering the color components is measured.
As the first method, for example, the image forming apparatus disclosed in Japanese Patent Laid-Open No. 10-213940 is known. The image forming apparatus disclosed in Japanese Patent Laid-Open No. 10-213940 detects the distance between transfer positions of the images of different color components to perform correction based on the detected amount of displacement in the transfer positions. The apparatus detects the distance between the image formed by a base color component and the image formed by another color component by a sensor and determines the amount of displacement in the transfer positions of the images of the color components based on the detected distance, to correct color misregistration.
As the second method, the image forming apparatus disclosed in Japanese Patent Laid-Open No. 2000-81744 is known. The image forming apparatus disclosed in Japanese Patent Laid-Open No. 2000-81744 measures the density of the multi-color image formed by layering the images of different color components and corrects color misregistration such that the density is that obtained when the images of the color components are precisely aligned. This image forming apparatus repeatedly forms a plurality of images of the same shape for each color component in order to improve the correction accuracy. Specifically, a plurality of images having the same linear shape are formed for each color component, and the density of each multicolor line image is detected by a sensor in order to obtain the alignment state of the line images of different color components. It is then assumed that the line images of the color components are accurately aligned when the density of the multi-line image detected by the sensor is in the range of a predetermined density. Then, color adjustment is performed by providing correction such that the images are formed in the accurately aligned state.
The image forming apparatus disclosed in Japanese Patent Laid-Open No. 10-213940 that uses a sensor for detecting transfer positions of the images of different color components to obtain displacement in the transfer positions of the images, however, had a problem such that a sensor with high detection accuracy must be used to detect small displacements in transfer positions. Color adjustment requires accuracy of several microns. Incorporation of such a sensor has caused a problem of increased cost.
The image forming apparatus disclosed in Japanese Patent Laid-Open No. 2000-81744 needs to shift the adjustment value line by line over the entire area of image color adjustment to obtain an adjustment value at which a base image and a color component image to be a subject for adjustment are perfectly aligned. This requires detection of the density used at color correction for the entire area of the image color adjustable range, which disadvantageously increases the time required for color adjustment. Moreover, if the time required for adjustment is made shorter, the image color adjustable area cannot be so widened. In particular, color misregistraion is caused by various reasons such as the temperature and moisture in the image forming apparatus, worn-out parts, and replacement of parts. Accordingly, besides at the time of factory shipment, regular corrections must be made even after delivery by a maintenance person or a user on the field. Thus, development of an image forming apparatus that can simply correct color misregistration with high accuracy has been requested.
According to the method of using a sensor to detect the density of the alignment state for the base image and the image of a color component to be a subject for adjustment, the sensor detects reflection light from the base image and the color component image that are formed on a transfer belt, whereas it detects reflection light from the transfer belt itself for the portion where no base image or image for adjustment are formed. There is a small difference between the reflection light from the transfer belt itself and the reflection light from the portion where images are formed. This may be the cause of deterioration in detection accuracy by the sensor, depending on the ratio of the image forming region to non-image forming region (the transfer belt itself) that are included in the sensor detecting area.